1. Field of the Invention
The present invention relates to a high heat conductive body which can attain efficient heat radiation with ease. The present invention further relates to a wiring base substrate for integrated circuits, fitted with such a high heat conductive body. The present invention still further relates to a method of producing such a high heat conductive body and a wiring base substrate fitted with the same.
2. Description of the Prior Art
In case the emission of heat from an integrated circuit chip is small, a wiring base substrate usually used for installation of such an integration circuit chip is of the kind in which the circuit wiring is simply formed between the insulation layers or on the exterior surface thereof.
However, in accordance with an increasing tendency that the integrated circuit chip becomes larger in size, higher in integration, faster in operation, and larger in power, the emission of heat from the integrated circuit chip in operation increases. Since such heat may be causative of malfunction of the integrated circuit and thus lowers the reliability, there is an increasing demand for a wiring base substrate having a high heat radiating property or function for radiating the heat coming from the integrated circuit chip rapidly.
Further, in case the material forming the integrated circuit chip and the material forming the wiring base substrate are different in coefficient of thermal expansion from each other, there arises a difference in thermal expansion in response to the emission of heat by the integrated circuit. By such a difference in thermal expansion, such a case may be caused in which the integrated circuit chip is subjected to a stress and damaged thereby or the integrated circuit chip is peeled off from the place where it is fixedly attached to the wiring base substrate, thus lowering the reliability of an associated electronic device.
In order that the reliability is maintained unchanged even if the emission of heat increases, it is required that the material for a wiring base substrate have a thermal expansion efficiency close to that of the material for the integrated circuit chip.
The following methods are known for attaining high heat radiation.
One known method is, as shown in FIG. 22, to attach heat radiating fins "F" to the bottom surface of the wiring base substrate 101.
Another known method is, as shown in FIGS. 23 and 24, to attach to the heat insulation layer 103 of the wiring base substrate 101 a heat radiating member or element "S" which is so-called a heat spreader or a heat slug and made of a high heat conductive material such as a Cu-W alloy.
In the meantime, in FIGS. 22-24, 102 is an integrated circuit chip, 104 is input/output terminal, 105 is a sealing cover, and 110 and 111 are joining layers such as brazing layers.
A further method is to form the insulation layer of the wiring base substrate from a high heat conductive material such as aluminum nitride in place of alumina usually used.
On the other hand, fin order to increase the reliability of an electronic device, it has been proposed to form the insulation layers of the wiring base substrate from a material the coefficient of thermal expansion of which is close to that of silicon which is a representative of all materials for integrated circuits such as aluminum nitride, mullite, glass ceramic, etc.
However, the above described conventional methods are encountered by the following problems.
Firstly, the material for a heat emitting member such as a heat spreader or heat slug needs to be close in coefficient of thermal expansion to that of the wiring base substrate material and the integrated circuit material since if not the joining of them is difficult. So, copper which is larger not only in heat conductivity but in coefficient of thermal expansion cannot be used. Accordingly, it is obliged to use a Cu-W alloy which approximates in coefficient of thermal expansion to alumina.
However, the Cu-W alloy is costly since it needs be produced by a difficult infiltration process and further since it has a difficulty in cutting.
Secondly, in order to attach the radiating fins or heat slugs made of metal to the wiring base substrate, a difficult ceramic-metal brazing technique is necessitated. Further, since a ceramic body and metallic body cannot be brazed together directly, it is necessitated that a metalizing layer is firstly formed on the wiring base substrate, then an electroplating layer is formed on the metalizing layer, and thereafter the brazing is performed.
This is because the ceramic material for the wiring base substrate has an insulating property and is not electrically conductive, so an electroplating layer cannot be formed on the surface of the wiring base substrate by an electroplating process.
Accordingly, a difficult process such as a process for firstly performing printing and baking of metallic paste to form a metalizing layer and then applying electroplating to the metalized layer is necessitated.
Further, in case the heat slug "S" is fitted as shown in FIG. 24, formation of the metalized layer or defective brazing may cause a crack or cracks at the joint 110 between the heat slug and the ceramic substrate or insufficiency in fitting, thus causing a problem on the reliability such as one related to the air tightness of the wiring base substrate.
Thirdly, aluminum nitride has a high heat conductive property and is therefore hopeful as a material in place of alumina but its raw material cannot be obtained at low price and further the technique of manufacturing a substrate therefrom is not established, resulting in a high cost and an unstable property that the material of itself is easily affected by water.
Further, for the joining of the radiating fins, integrated circuits, etc., it is indispensable to form a metallizing layer from metallic paste.